Scene 3 does that because default doesn't work that way. If you wanted the namespace stripped then you'd do something like

sub base :Path('') {
...
}

which will be a path action anchored at /greeting and as such should see args of ('hello', 'world').
Scene 4, I'm not entirely sure - I wonder if there's a default in your app class picking up the result or similar ...
- Matt S Trout (mst on irc.perl.org)